Electrochemical fuel cells generate electrical energy by converting chemical energy derived from a fuel directly into electrical energy by the oxidation of the fuel in the cell. A typical fuel cell includes an anode, a cathode and an electrolyte. Fuel and oxidant are supplied to the anode and cathode, respectively. At the anode, the fuel permeates the electrode material and reacts at the anode catalyst layer to form cations, which migrate through the electrolyte to the cathode. At the cathode, the oxygen containing gas supply reacts at the cathode catalyst layer to form anions. The anions formed at the cathode react with the cations to form a reaction product. The fuel cell generates a useable electric current and the reaction product is removed from the cell.
In electrochemical fuel cells employing hydrogen as the fuel and oxygen containing air (or pure oxygen) as the oxidant, a catalyzed reaction at the anode produces hydrogen cations from the fuel supply. An ion exchange membrane facilitates the migration of hydrogen ions (protons) from the anode to the cathode. In addition to conducting hydrogen cations, the membrane isolates the hydrogen fuel stream from the oxidant stream comprising oxygen containing air. At the cathode, oxygen reacts at the catalyst layer to form anions. The anions formed at the cathode react with the hydrogen ions that have crossed the membrane to form liquid water as the reaction product.
The anode and cathode reactions in such fuel cells is shown in equations (1) and (2) below: EQU Anode reaction H.sub.2 .fwdarw.2.sup.+ +2e.sup.- ( 1) EQU Cathode reaction 1/20.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O(2)
Solid polymer fuel cells generally contain a membrane electrode assembly ("MEA") consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrodes formed of porous, electrically conductive sheet material. The electrodes are typically formed of carbon fiber paper, and are generally impregnated or coated with a hydrophobic polymer, such as polytetrafluoroethylene. The MEA contains a layer of catalyst at each membrane/electrode interface to induce the desired electrochemical reaction. A finely divided platinum catalyst is typically employed. The MEA is in turn disposed between two electrically conductive plates, each of which has at least one flow passage engraved or milled therein. These fluid flow field plates are typically formed of graphite The flow passages direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes.
In a single cell arrangement, fluid flow field plates are provided on each of the anode and cathode sides. The plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant to the respective anode and cathode surfaces, and provide channels for the removal of water formed during operation of the cell.
Two or more fuel cells can be connected together in series or in parallel to increase the overall power output of the assembly. In such arrangements, the cells are typically connected in series, wherein one side of a given plate serves as an anode plate for one cell and the other side of the plate is the cathode plate for the adjacent cell. Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack, and is usually held together by tie rods and end plates. The stack typically includes feed manifolds or inlets for directing the fuel (substantially pure hydrogen, methanol reformate or natural gas reformate) and the oxidant (substantially pure oxygen or oxygen containing air) to the anode and cathode flow field channels. The stack also usually includes a feed manifold or inlet for directing the coolant fluid, typically water, to interior channels within the stack to absorb heat generated by the exothermic reaction of hydrogen and oxygen within the fuel cells. The stack also generally includes exhaust manifolds or outlets for expelling the unreacted fuel and oxidant gases, each carrying entrained water, as well as an outlet manifold for the coolant water exiting the stack.
Perfluorosulfonic ion exchange membranes, such as those sold by DuPont under its Nafion trade designation, must be hydrated or saturated with water molecules for ion transport to occur. It is generally believed that such perfluorosulfonic membranes transport cations using a "water pumping" mechanism. Water pumping involves the transport of cations in conjunction with water molecules, resulting in a net flow of water from the anode side of the membrane to the cathode side. Thus, membranes exhibiting the water pumping mechanism can dry out, especially on the anode side, if water transported along with hydrogen ions is not replenished. Such replenishment typically occurs by humidifying the hydrogen containing fuel stream prior to introducing the fuel stream into the cell. Similarly, the oxygen containing oxidant stream is typically humidified prior to introducing the oxidant stream into the fuel cell to prevent the membrane from drying out on the cathode side.
A new type of experimental perfluorosulfonic ion exchange membrane, sold by Dow under the trade designation XUS13204.10, does not appear to significantly exhibit the water pumping mechanism in connection with the transport of hydrogenions across the membrane. Thus, the transport of water molecules across the Dow experimental membranes does not appear to be necessary for the transport of hydrogen ions as in the Nafion type membranes. Despite the apparent absence of water pumping, however, the Dow experimental membranes still appear to require hydration to effect hydrogen ion transport.
In fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the fuel can be supplied in the form of substantially pure hydrogen or as a hydrogen containing reformate as, for example, the product of the reformation of methanol and water or reformation of natural gas. Similarly, the oxidant can be supplied in the form of substantially pure oxygen or oxygen containing air. The fuel cells are typically flooded with fuel and oxidant at constant pressure. Pressure is generally controlled by a pressure regulator at the source of the reactant. When an electrical load is placed on the circuit connecting the electrodes, fuel and oxidant are consumed in direct proportion to the electrical current drawn by the load.
When using substantially pure reactants, the unconsumed reactants exiting the fuel cell stack are recirculated to minimize waste. Water in the gas streams exiting the fuel cells is accumulated in a separator or knockout drum, where the water can be recirculated and used as a coolant or drained from the system. The fuel stream exiting the stack generally contains water from the humidification of the fuel stream prior to its introduction into the fuel cell stack. The oxidant stream exiting the stack generally contains product water generated at the cathodes of the fuel cells in addition to the water from the humidification of the oxidant stream. After removal of water from the stream, the stream is recirculated and merged with the source gas stream prior to the inlet of the fuel cell stack. The flow rate of recirculated gas is usually controlled by a compressor.
When using dilute reactants, such as reformate or air, the unconsumed reactant streams exiting the fuel cell stack are generally not recirculated. However, water in such dilute gas streams is generally removed in a 5 separator or knockout drum and then drained. The partially depleted reactant streams are generally vented to the atmosphere.
As discussed above, hydrogen ion conductivity through ion exchange membranes generally requires the presence of water molecules. The fuel and oxidant gases are therefore humidified prior to introducing them to the cell to maintain the saturation of the membranes within the MEAs. Ordinarily, the fuel and oxidant gases are humidified by flowing each gas on one side of a water vapor exchange membrane and by flowing deionized water on the opposite side of the membrane. Deionized water is preferred to prevent membrane contamination by undesired ions. In such membrane based humidification arrangements, water is osmotically transferred across the membrane to the fuel and oxidant gases. Nafion is a suitable and convenient humidification membrane material in such applications, but other commercially available water exchange membranes are suitable as well. Other nonmembrane based humidification techniques could be employed, such as exposing the gases directly to water in an evaporation chamber to permit the gas to absorb evaporated water.
It is generally preferred to humidify the fuel and oxidant gases at, or as close as possible to, the operating temperature and pressure of the fuel cell. The ability of gases such as air to absorb water vapor varies significantly with changes in temperature, especially at low operating pressures. Humidification of the air (oxidant) stream at a temperature significantly below fuel cell operating temperature could ultimately dehydrate the membrane. Consequently, it is preferable to integrate the humidification function with the active portion of the fuel cell stack, and to condition the fuel and oxidant streams to nearly the same temperature and pressure as the active section of the stack. In such an integrated arrangement, the coolant water stream from the active section, which is at or near the cell operating temperature, is used as the humidification water stream. Similarly, the fuel and oxidant streams are typically directed via manifolds or headers through the active section to condition each to cell temperature prior to introducing them to the humidification section.
In addition to integrating the coolant water stream of the active section with the humidification water stream of the humidification section, it is also advantageous to integrate the fuel cell product water stream with the coolant stream, and thereby use the product water generated electrochemically in the fuel cell stack to regulate the temperature of the stack. In this regard, the use of product water as the coolant avoids the need to provide a separate external source of coolant fluid, since the water generated by the cell is itself a suitable coolant fluid. The use of product water as the coolant fluid is also advantageous during start up, when the relatively warm product water stream can be used to rapidly bring the active section up to operating temperature.